How to select outermost vertices in a shape like this?
Select Boundary Loop.

First, A Select All
Then in 2 Edge select mode,
Select &gt; Select Loops &gt; Select Boundary Loop
At this stage an option is to PS Separate Selected, to make the boundary edge a new object... or

dissolve (or delete) internal verts to end up with one big ngon state
1 Vert select mode,
CtrlI Select &gt; Invert
Then XD Dissolve Vertices.